(1) Field of the Invention
The present invention relates to an improved method for etching a surface of a material or workpiece using a radiofrequency wave RF, particularly a microwave, plasma reactor. In particular, the present invention relates to an improved method for plasma etching, including electron-cyclotron-resonance (ECR) plasma etching at pressures below about 10 torr of a surface of a diamond wafer to provide increased etch rate and improved uniformity of etching. This result is achieved particularly by changing resonant radiofrequency modes in the plasma reactor over time dependent upon the surface geometry of the particular wafer.
(2) Prior Art
As-grown, free-standing diamond wafers contain various degrees of surface roughness, bowing, and thickness variations which vary from wafer to wafer. Consequently, post-processing is required to achieve flat surfaces of uniform thickness. Presently, the diamond wafers are polished to a uniform thickness using mechanical means. The mechanical method achieves good results, but is slow. Various alternative methods have been investigated for diamond post-processing including plasma etching (S. J. Pearton, A. Katz, F. Ren, and J. R. Lothian, Electronics Letters, 28, 822, (1992)), ion beam irradiation, (N. N. Efremow, M. W. Geis, D. C. Flanders, G. A. Lincoln, and N. P. Economou, J. Vac. Sci. Technol. B, 3, 416, (1985)) laser planing, (M. Yoshikawa, SPIE Diamond Optics III 1325 210-217 (1990)) and diffusional reactions with hot metals (S. Jin, J. E. Graebner, T. H. Tiefel and J G W. Kammlott, Diamond and Related Materials, 2, 1038, (1993)). Pearton et al reported higher removal rates using plasma etching however the issue of uniformity was not addressed.
As-grown diamond wafers have radial thickness variations but have angular symmetry which vary from wafer to wafer. Some wafers are thickest in the center, some are thickest at the edges, and some are thickest at some radial point between the edge and center. Therefore, it is a significant challenge to achieve a high diamond removal or etch rate uniformly over large areas where there is a lack of uniformity of the surface irregularities for a given wafer and also from wafer to wafer. There remains a need for a method of polishing or etching a material which is fast, which allows for uniform etching over large diameters and which allows the etch profile to be customized for each material to be etched.
U.S. Pat. No. 5,081,398 to Asmussen et al describes an apparatus which can be used in the method of the present invention. Related patents are U.S. Pat. Nos. 5,311,103; 4,507,588 and 4,727,293 to Asmussen et al. Also, U.S. Pat. No. 4,691,662 to Roppel et al describes such apparatus. The method of the present invention uses the plasma technology described in these patents.